1. Field of the Invention
The present invention relates to air foil journal bearings for supporting shaft of a rotation system, and more particularly to a smart foil journal bearing with piezoelectric actuators, which is capable of increasing its load-carrying capacity and eliminating its operation instability by controlling the damping of vibrations at critical speed.
2. Description of the Prior Art
In general, a air foil journal bearing employs air as lubricating material instead of lubricating oil, and is a mechanical element suitable for a clean mechanical system. Such a foil journal bearing has a relatively wide area for supporting a rotating member in comparison with other bearings, thus having high reliability. Therefore, the foil journal bearing can be utilized as a supporting bearing for small, general aerial gas turbine engines, turbo expanders, various aerospace auxiliary power units, land transportation means, vapor cycle centrifugal compressors, and various commercial air compressors.
The air foil bearing should be designed to have load-carrying capacity and stability, and to withstand wear due to unstable contact between a shaft and a bearing during its start and stop.
FIG. 1 is a sectional view of a conventional multi-leaf foil journal bearing. A plurality of slots 32 are formed on the inner surface of a bearing housing 30 while being spaced apart from one another. A plurality of foils 20 are fixed by the slots 32, and support a shaft 10. The conventional multi-leaf foil journal bearing can increase hydrodynamic stiffness by increasing the thickness of the foils 20. However, the conventional multi-leaf foil journal bearing is problematic in that the life span of the bearing is shortened by wear between the shaft 10 and the foils 20 due to relatively increased start torque. Additionally, the vibration control of the shaft 10 depends upon only the repulsive force of the foils 20, so that the load-carrying capacity of the bearing is restricted and the effective control the vibrations is difficult thereby causing the shaft 10 to be unstable when the shaft rotates fast This creates unstable contact between the foil journal bearing and the shaft 10.
FIG. 2 is a sectional view of a conventional bump foil journal bearing in which a top foil 40 and a bump foil 42 are positioned on the inner surface of a bearing housing 30 and support a shaft 10. The bump foil journal bearing can increase load-carrying capacity by the great stiffness of the bump foil 42. However, gas in the bump foil 42 that is elastically deformed by the load exerted on the shaft 10 leaks in axial direction of the shaft, thereby weakening the damping capacity of the bump foil journal bearing and, accordingly, causing the control of vibrations at a high-speed region to be difficult.
Accordingly, the main object of the present invention is to provide a foil journal bearing, in which a plurality of piezoelectric actuators are positioned to support the exterior surface of a foil to increase stiffness against shaft-supported load and damping capability at a high-speed region, thereby improving its load-carrying capacity, enabling the reduction of the vibration amplitude of the shaft, and eliminating the instability of the shaft at critical speeds.
In order to accomplish the above object, the present invention provides a foil journal bearing, which is provided at an interior surface of its bearing housing with a plurality of foils or a top foil for supporting a rotating shaft, comprising: a plurality of piezoelectric actuator devices embedded in an inside portion of the bearing housing while coming into contact with and supporting the foils or the foil to increase stiffness and damping coefficient of the bearing at critical speeds of the rotating shalt wherein each of the piezoelectric actuator devices is comprised of a piezoelectric actuator inserted into one of dimples formed on the interior surface of the bearing housing and provided with at least a piezoelectric actuating element and an elastic spring interposed between a bottom of the piezoelectric actuator and a bottom of the dimple, so as to increase stiffness and damping coefficient of the bearing by application of voltage to the piezoelectric actuator.
The foil journal bearing in accordance with the present invention has also a thrust which is a force of the direction of the shaft and effectively supports the loaded shaft. Therefore, the foil journal bearing has much more load-carrying capabilities to any direction. (i.e., to the radial and thrust directions).